Prior art sliding contact bearings in aircraft have presented excessive weight and poor overall performance problems. Sliding bearings used in landing gear, for example, are relatively large and take a substantial load. Weight reduction in such bearings can only be achieved by substituting lower density or more efficient materials, that is, smaller bearing envelopes.
Many of the present bearings are made of steel and are very heavy compared with titanium, for example. Titanium is a very desirable metal because of its lightweight but it had not been used successfully in metal-to-metal sliding surface contact due to severe galling problems. In the prior art no coatings or lubricants had been developed to solve this metal-to-metal bearing problem.
Chromium oxide coating has been used on titanium spherical bearings with a Teflon coated race in hydrofoil applications and in the wing pivot bearing on an F-14 airplane. Although chromium oxide is known to be brittle, it has been found to be satisfactory as a coating and used in a Teflon race because the lubrication is permanent and in situations where there are no significant shock loads which would tend to chip the brittle chromium oxide. That is, there is no risk of losing lubrication with a Teflon race but in situations where a bearing could become grease starved or subject to shock loads, it was considered that chromium oxide would not be a satisfactory coating for titanium or other metals when used against a metal race, including copper or copper alloys. Landing gear trunnion bearings on a large airplane obviously take significant shock loads and, therefore, it was considered that chromium oxide would be too brittle to be used as a coating on a titanium bearing, the titanium being considered primarily as a weight saving material in contrast to the steel bearings being used.
A search of the patent literature discloses the use of titanium bearings. For example, U.S. Pat. No. 3,724,919 teaches the use of a titanium allow for a low friction, seawater-lubricated bearing in conjunction with a high density polyethylene race, but did not suggest the use of chromium oxide as a coating for titanium.
U.S. Pat. Nos. 4,309,474; 4,043,623 and British Pat. No. 1,023,007 teach the use of chromium oxide as a surface coating for bearing surfaces but titanium was not one of the alloys included in these patents.
The following patents disclose subject matter of general interest:
U.S. Pat. No. 2,934,480 Slomin, PA1 U.S. Pat. No. 3,143,383 Bamberger et al, PA1 U.S. Pat. No. 3,297,552 Gisser et al, PA1 U.S. Pat. No. 3,418,028 Watson et al, PA1 U.S. Pat. No. 3,391,287 Vogt, PA1 No. 541,580 Great Britain.